As is well known in the art, wheel lock-up and vehicle skidding can be caused by severe slippage between the wheel and the driving surface. In many cases, lock-up increases required stopping distances and reduces directional control of the vehicle.
Such problems have been generally addressed with the advent of antilock brake systems (ABS). A typical ABS is designed to identify an excessive wheel slip condition by comparing the measured velocity of a wheel to a reference speed, which is an estimate of the true vehicle speed based on current and previous values of individual wheel velocities. If the velocity of a particular wheel is significantly less than the reference speed, then that wheel is determined to be experiencing excessive slip. In response, hydraulic pressure actuating a corresponding brake is modulated to reduce brake torque, thereby reducing braking force between the wheel and driving surface which, in turn, reduces wheel slip.
In practice, ABS first isolates existing brake fluid in an individual wheel brake from increasing brake fluid pressure in the master cylinder in order to hold pressure in the brake constant. ABS then dumps fluid from the brake to reduce pressure therewithin. Thereafter, ABS typically holds pressure in the brake constant for a selected amount of time.
After a period of constant pressure following pressure reduction, pressure is then increased until excessive wheel slip occurs again. The resulting cycle of decreasing, maintaining, and then increasing pressure is repeated until excessive slip no longer occurs. The specifics of this brake pressure cycle depend on the particular algorithm employed within the ABS logic control unit, along with vehicle characteristics and the driving surface conditions encountered at the time of braking.
One parameter which represents driving surface conditions is the coefficient of friction, commonly denoted by mu. Two classes of surfaces can be defined qualitatively in terms of mu. A high mu surface is one wherein relatively good braking ability is possible, such as dry asphalt. A low mu surface is one wherein relatively poor braking ability is possible, such as a snow or ice-covered road, or wet asphalt.
In relative terms, the coefficient of friction, mu, can also be expressed as a variation from a current or reference mu. Specifically, a higher mu surface is a surface whose mu is greater than the reference and a lower mu surface is a surface whose mu is less than the reference.
Vehicles with hard-coupled, four-wheel drive (4 wd) systems require special control by the ABS. For example, one known prior art system initiates the application of brake pressure to all the wheels during an ABS event, and then removes brake pressure from the rear wheels if a low mu or mid mu surface is detected based on external sensors. In this system, torque transfer problems, as is known in the art, occur when brake pressure is applied to the rear wheels while the vehicle is braking on a low mu or mid mu surface before the system determines the surface coefficient.
A second known prior art system controls the ABS braking event when in four-wheel drive by utilizing only the front channels and maintaining a low brake pressure on the rear brakes. However, this is not desirable on a high mu surface since brake torque produced by the front brakes is insufficient to cause significant slip when all four wheels are on high mu, resulting in the under utilization of the surface adhesion.
Therefore, a need exists for an anti-lock brake system to detect braking on a high mu surface during four-wheel drive in order to initiate full rear pressure control.